Refactor email handling for multiple recipients

Refactored the `Email` and `OutgoingEmailEvent` classes to replace the `Recipient` property with a `Recipients` collection, enabling support for multiple recipients. Updated all related test cases, including `EmailSenderTests`, `EmailSenderUrlOverloadTests`, and `OutgoingEmailPublisherTests`, to reflect this change.

Moved the `EmailAccountDto` class and its references from the `DigitalData.MessagingService.Application.Common.Dtos` namespace to the `DigitalData.MessagingService.Publisher.Abstraction` namespace for better code organization. Updated `using` directives across affected files.

Removed unused `using` directives and updated the `Email` class's `ToEvent` method to map the new `Recipients` property. Adjusted test assertions to validate collections instead of single recipient strings.

tests/DigitalData.MessagingService.Tests/Integration/EmailSenderTests.cs

@@ -77,7 +77,7 @@ public sealed class EmailSenderTests
     {
         var email = new Email
         {
-            Recipient = "hakanttek@gmail.com",
+            Recipients = ["hakanttek@gmail.com"],
             Subject = "EmailSender.Send Integration Test",
             Body = "<p>Sent via EmailSender static client.</p>",
             IsHtml = true,
@@ -93,7 +93,7 @@ public sealed class EmailSenderTests
     {
         var email = new Email
         {
-            Recipient = "hakanttek@gmail.com",
+            Recipients = ["hakanttek@gmail.com"],
             Subject = "Plain Text Test",
             Body = "This is a plain text email.",
             IsHtml = false